1. Field of the Invention
The present invention relates to rotary machines and particularly to sealing of a medium flow path within the machine.
2. Description of the Prior Art
The design and construction of efficient rotary machines, and of gas turbine engines in particular, has historically required careful confinement of the working medium gases to the flow path of the machine to preserve aerodynamic performance and to protect the internal components of the machine from thermal degradation.
Typical construction details in a region radially inward of the working medium flow path of a gas turbine engine are shown in U.S. Pat. No. 3,515,112 to Pettengill entitled "Reduced Clearance Seal Construction." In a Pettengill type construction the radially inward ingestion of working medium gases into the internal regions of the machine is prevented by flowing air radially outward between the stator or stationary element and the rotor or rotating element of the machine. The air flowed outwardly is termed purge air and is supplied to the cavity at a pressure greater than the pressure of the local working medium gases in the flow path. The rate of flow of the purge air through the cavity is set by the minimized combination of pressure differential and flow area between the purge supply and the flow path. For example, if the minimized flow conditions in the Pettengill construction occur across the labyrinth seal, the rate of flow across the seal will establish the rate of flow through the cavity. Similarly, if the minimized conditions of pressure differential and area occur across the narrow passage between the relatively rotating components at the disk rim, the flow rate through the cavity will be restricted by the flow rate through the passage.
Within the cavity the purge air adjacent the rotating member is pumped radially outwardly in response to frictional forces between the air and the radially extending surfaces of the rotor. If the pumping rate exceeds the rate at which purge air is supplied through the labyrinth seal, a circulation zone is established within the cavity. The excess of pumped air over purge air is forced across the passage leading to the working medium flow path and radially inward along the stationary member. As the circulating air travels across the passage, a portion of the working medium gases is ingested and circulated with the cavity air. As this occurs, the temperature of the air within the cavity becomes elevated and the durability of the local components becomes adversely effected.
New concepts are continually sought within the rotary machinery art to minimize the performance losses inherently imposed upon the machine by flowing substantial amounts of purge air between the relatively rotating components to prevent ingestion of the working medium gases.